1. Field of the Invention
The present invention relates to a metal oxide electrode material and an electrochemical capacitor using the same, and a method for producing the same. More particularly, the present invention relates to a metal oxide electrode material substituting a sublattice location of metal for one or more kinds of different metals, an electrochemical capacitor using the same, and a method for producing the same in metal oxides constituting an electrochemical capacitor electrode.
2. Description of the Related Art
In general, an electronic device called a capacitor as a device storing electricity in a physical mechanism without a chemical reaction or a phase change takes charge of collecting and discharging the electricity to stabilize electrical flow in a circuit. The capacitor has a very short charging/discharging time, a long lifespan, and very high power density, but since the capacitor has very low energy density, the capacitor is limitative to be used as an energy storage device.
On the contrary, a rechargeable battery as a device capable of storing high-density energy is used as an energy storage medium of portable electronic apparatuses such as a notebook, a cellular phone, a PDA, etc. Recently, a research on a lithium ion battery has been actively in progress.
An electrochemical capacitor is being in the limelight as a storage medium used for an electronic apparatus requiring high energy density and high power density by expressing a medium characteristic between two devices. FIG. 1 shows a correlation between energy density and power density of energy storage devices and in FIG. 1, the electrochemical capacitor expresses a medium characteristic between a general capacitor and a general rechargeable battery.
The electrochemical capacitor is referred to as a super capacitor, an electrical double layer capacitor, an ultracapacitor, or the like. With a latent application probability of the electrochemical capacitor as the energy storage medium in various fields such as wind power generation, a hybrid electric vehicle, an electric vehicle, etc., world countries are enormously interested in the electrochemical capacitor.
The most important part of the super capacitor is an electrode material, which should have a large specific surface area, have high electroconductivity having minimum voltage drop distribution in an electrode, and have an electrochemical stabilization characteristic under a predetermined potential, and in addition, it should be low in cost for commercialization.
The super capacitor is classified into three types in accordance with an electrode and a mechanism primarily used as shown in FIG. 2. Three types include (1) an electric double layer capacitor generally using activated carbon as an electrode and having charge absorption of an electrical double layer as a mechanism, (2) a metal oxide electrode pseudocapacitor (alternatively, redox capacitor) using transition metal oxide or conductive polymer as the electrode material and having pseudo-capacitance as the mechanism, and (3) a hybrid capacitor having a medium characteristic between the capacitors.
A basic structure of the super capacitor is constituted by a porous electrode, an electrolyte, a current collector, and a separator. The super capacitor has an electrochemical mechanism in which ions in electrolyte solution move on an electric field and absorb on the surface of the electrode to generate electricity by applying a voltage of several volts to both terminal of a unit cell as an operation principle.
In case of the activated carbon electrode material, since specific capacitance is in proportion to the specific surface area, energy density according to high capacity of the electrode material is increased by granting porosity. The electrode is manufactured by making a carbon electrode material, a carbon conductive material, and a polymer binder in a slurry form and applying them to the current collector. It is important to reduce a contact resistance while increasing an adhesion force to the current collector and reduce an internal contact resistance between activated carbons by varying the type and ratio of the binder, the conductive material, and the electrode material.
In case of the metal oxide electrode material, since transition metal oxide which is advantageous in capacity has resistance lower than activated carbon, it is possible to manufacture a super capacitor having high-output characteristics and in recent years, when amorphous oxide is used as the electrode material, it is reported that non-capacitance is remarkably increased. Recently, it has been reported that the metal oxide electrode material shows power and energy densities better than the known electrode using only the transition metal oxide by oxidizing only the surface using nitride having electrical conductivity better than the oxide by P. N. Kumta et al.
Meanwhile, in case of the hybrid capacitor for combining the advantages of the capacitors, a research to increase operating voltage and improve energy density by using an asymmetric electrode is actively in progress. The hybrid capacitor is a capacitor that maintains power characteristics thereof by using a material having an electrical double layer characteristic, that is, carbon for one electrode and improves overall energy of a cell by using an electrode showing an oxidation-reduction mechanism having a high-capacity characteristic.
In particular, in recent years, the super capacitor serves as a braking regenerative power supply of an electrical vehicle, such that a research to maintain an advantage an EDLC and improve energy density has been actively in progress by an increase in capacity of an electrode material and an increase in density of an electrode, and modification of a manufacturing process.
As a result, the inventor has developed a metal oxide electrode increasing low electrical conductivity of the known metal oxide and having remarkably increased non-capacitance and energy density and a high-power and high-density super capacitor using the same.